GROUP III. PTERIDOPHYTA : 



427 



dichotonious. In some few species, however, the branches have radial 

 symmetry (e.g. S. spinosa). 



At the points at which the normal branching takes place, leafless branches, 

 termed rliizophores, are in some species developed in a plane at right angles to 

 that of the normal branching ; thus in S. Kraussiana they arise singly on the 

 upper surface of the stem at the points of normal branching, whilst in S. 

 niartensii two are developed at each such point, one on the upper and one on 

 the lower surface. The direction of growth of the rhizophore is such that the 

 apex eventually penetrates into the soil, when roots arise from it and it ceases 

 to grow. 



These organs have been regarded as roots, and are sometimes so designated 

 still. But in view of the important morphological facts that the rhizophore is 

 of exogenous origin like the leafy branches ; that it has no root-cap, whilst the 

 true roots of Selaginella have one ; and finally, that sometimes a rhizophore will 

 develope leaves and even cones, the probability is that they are modified 

 branches comparable with the root-like branches of the Psilotacese. 



The leaves can be readily distinguished as either foliage-leaves or sporo- 

 phylls. A characteristic feature in their morphology is the development of a 

 small ligule on the upper surface of each leaf near its base. 



The foliage-leaves are simple, small, sessile, and rather numerous. Those 

 borne on the radial branches are all alike, and are arranged spirally ; the 

 bilateral branches show remarkable heterophylly, there being leaves of two 

 sizes in decussate pairs, each pair con- 

 sisting of one large and one small leaf ; 

 when the branch bearing these two kinds 

 of leaves is dorsiventral, the four rows of 

 leaves show displacement with the result 

 that the two rows of small leaves are 

 borne on the upper surface of the branch, 

 and the two rows of larger leaves are 

 borne infero laterally (Fig. 277). 



The sporophylls are generally collected 

 into more or less distinct cone-like flowers ; 

 they do not differ materially from the 

 foliage-leaves, and, like them, may be all 

 of one size or of two sizes. 



The sporangia are shortly stalked and 

 unilocular; they aris singly from a group 

 of superficial cells of the stem just above 

 the insertion of each sporophyll ; the wall, 

 when mature, consists of two layers of 

 cells ; the archesporium probably consists of a single row of cells, and is entirely 

 sporogenous. 



There are two kinds of sporangia, macrosporangia and microsporangia, 

 distinguished by the kind of spores which they produce, and by their size. 

 The macrosporangia each give rise to generally four (sometimes 2 or 8) relatively 

 large macrospores ; the microsporangia each give rise to a conside:able number 

 of microspores. 



FIG. 277. Selaginella helvetica (nat. 

 size) : s the upright fertile shoot, with 

 sporangia in the axil* of the leaves. 

 On the procumbent sterile shoots, the 

 leaves on the under side (u) are larger 

 than those on the upper side (o). 



